By Syed Asad Raza Shah Naqvi, Athar Mahmood, Safura Bibi, Saba
Phytoremediation is energy efficient, aesthetically pleasing method of remediating sites with low to moderate levels of contamination that it can be implemented in conjunction with other effective traditional methods as finishing step to the remedial process. Specially selected or engineered plants are used in the process.
An ideal Phytoremediation possess high tolerance to heavy metals, should be able to either degrade or concentrate the contaminant in their extensive root systems exhibit the capacity to absorb large amount of contaminant from the soil that preferably accumulate in the above ground parts, exhibit fast growth rates coupled with high levels of biomass and easy harvest ability and there should be minimal secondary waste that requires disposal.
Why We Need Phytoremediation
Heavy metals are commonly defined as elements that have a density at least 5 times higher than of water. Their presence in the soil can be of natural and anthropogenic origin. Due to natural processes in the earth’s crust, the soil usually contains low concentrations of heavy metals. The concentration of heavy metals in the soil in unpolluted areas depends on the composition of the Earth’s crust. Low concentrations for most heavy metals are desirable, because of the essential importance for the living organisms. However, different anthropogenic activities lead to the increase of heavy metals concentration above the natural level.
The most common metals encountered in contaminated soil are: As, Cd, Cr, Pb, Hg, Ni, Zn, etc. As heavy metals are not biodegradable, they accumulate in the environment and enter the food chain. Excessive intake of heavy metals into living organism causes many harmful consequences; including death. Metal containing industrial effluents constitute a major source of metallic pollution of hydrosphere. Another means of dispersal is the movement of drainage water from catchment areas.
As well as the matter of human beings is concerned, Heavy metal toxicity can either be acute or chronic effects. Long-term exposure of the body to heavy metal can progressively lead to muscular, physical and neurological degenerative processes that are similar to diseases such as Parkinson’s disease, multiple sclerosis and Alzheimer’s disease, developmental retardation, several types of cancer, kidney damage, endocrine disruption, immunological, neurological effects and other disorders. The ongoing research works throw more light onto new insights and biochemical and molecular mechanisms involved in the development of pathological conditions in human. The way of heavy metal input into humans is inhalation, ingestion and direct and indirect contact.
Types of Phytoremediation
More than 400 plant species have been identified to have potential for soil and water remediation. Among them, Thlaspi, Brassica, Sedum alfredii H., and Arabidopsis species have been mostly studied.
There are several different types of phytoremediation mechanisms. These are:
- Rhizosphere biodegradation. In this process, the plant releasesnatural substances through its roots, supplying nutrients to microorganisms in the soil.
- Phyto-stabilization. In this process, chemical compounds produced by the plant immobilize contaminants, rather than degrade them.
- Phyto-accumulation (also called phyto-extraction). In this, plant roots absorbthe contaminants along with other nutrients. The contaminant mass is not destroyed but ends up in the plant shoots and leaves. This method is used primarily for wastes containing metals. At one demonstration site, water-soluble metals are taken up by plant species selected for their ability to take up large quantities of lead (Pb). The metals are stored in the plant aerial shoots, which are harvested and either smelted for potential metal recycling/recovery or are disposed of as a hazardous waste.
- Hydroponic Systems for Treating Water Streams (Rhizofiltration). Rhizofiltration is similar to phyto-accumulation, but the plants used for cleanupare raised in greenhouses with their roots in water. This system can be used for ex-situ groundwater treatment. That is, groundwater is pumped to the surface to irrigate these plants. Typically hydroponic systems utilize an artificial soil medium, such as sand mixed with perlite or vermiculite. As the roots become saturated with contaminants, they are harvested and disposed of.
- Phyto-volatilization. In this process, plants take up water containing organiccontaminants and release the contaminants into the air through their leaves.
Phytoremediation, being more cost-effective and fewer side effects than physical and chemical approaches, has gained increasing popularity in both academic and practical circles.
There is a need to understand metabolism or degradation of the approximately 200 000 secondary plant metabolites. If the structure and activity of a xenobiotic chemical are similar to that of a secondary metabolite, there is a good chance that a sustainable phytoremediation application is possible. Because some xenobiotic compounds may not be analogous to any secondary metabolite, some genetic engineering may be necessary to sustainably manage these unusual wastes. Plants seem to be the optimal organism in which to transplant microbial genes to achieve more mineralization of organic contaminants and mammalian genes that may have greater activity.
Authors : Syed Asad Raza Shah Naqvi, Athar Mahmood, Safura Bibi, Saba University of Agriculture Faisalabad